Separations of gas mixtures containing nitrogen are important industrial processes. The recovery of oxygen and/or nitrogen from air is practiced on a large scale. In the past, the primary method used for this separation was cryogenic distillation. More recently, pressure swing adsorption (PSA) processes are being used in applications which have smaller gas requirements. In PSA processes, compressed gas is fed through a bed containing an adsorbent material with a preference for one of the components of the gas to produce an exit stream enriched in the other components. A stream enriched in the adsorbed component can be obtained by desorption.
The use of crystalline zeolitic molecular sieves in PSA processes for air separation is well known. McRobbie in U.S. Pat. No. 3,140,931 claims the use of crystalline zeolitic molecular sieve material having apparent pore sizes of at least 4.6 Angstroms for separating oxygen-nitrogen mixtures at subambient temperatures. McKee in U.S. Pat. No. 3,140,932 specifically claims the Sr, Ba, or Ni ion exchanged forms of X-zeolite. McKee in U.S. Pat. No. 3,140,933 claimed the use of LiX-zeolite to separate oxygen-nitrogen mixtures at feed pressures between 0. 5 and 5 atm and at a temperature between about 30.degree. C. and -150.degree. C. Berlin, in U.S. Pat. No. 3,313,091 claims the use of SrX-zeolite at adsorption temperatures near atmospheric, and subatmospheric desorption pressures.
Formed adsorbent particles containing zeolites used for equilibrium air separation also typically contain about 20 wt % inert inorganic material. The purpose of this material is to bind the zeolite crystallites into an agglomerate having high physical strength and attrition resistance in order that the zeolite crystallites can be used in adsorption processing.
Several patents claiming zeolitic adsorbents for air separation have acknowledged the presence of this inert material. For example, Coe et al. in U.S. Pat. Nos. 4,481,018 and 4,544,378 demonstrated the improved performance of faujasite compositions containing divalent cations provided they were activated in such a way that a preponderance of the polyvalent cations were in the dehydrated/dehydroxylated state. They claim compositions containing at least 50 wt % faujasite, preferably 75 wt % faujasite, wherein the balance is selected from the group consisting of an A-type zeolite, an inert clay binder, other inert solid materials and mixtures thereof. However, there is no indication that there is any benefit in adsorptive performance to be gained by the use of binder.
Various other processes have been reported in the patent literature for achieving a zeolite in a final product contained in a matrix, where, in general, the zeolite component is present to the extent of only 10 to 20 wt %. However, these products are largely directed toward use in cracking catalysts. (D. W. Breck, Zeolite Molecular Sieves, 1974, p. 737).
Breck in U.S. Pat. No. 3,181,231 claims an agglomerate material comprised of zeolite molecular sieve particles and larger metal bodies, present in levels of 5 to 30% by weight, sintered to the outer surface of the zeolite crystals. The objective of his invention is to produce a zeolitic material having superior crush strength. He indicates that greater than 30 wt % metal bodies is not necessary, and, in fact, is undesirable. He states that it is preferred to use as little binder as possible to achieve the desired hardness, since excess binder reduces the adsorptive capacity. Breck makes no mention of a possible process performance advantage to be gained by the presence of such a high-heat-capacity binding material within the formed agglomerate.
Japanese Kokai 62 297,211-A2 discloses a porous body, presumably a monolith, containing 40-5 weight percent inorganic binder and 60-95 weight percent zeolite powder in general. It teaches that these bodies gave good results in O.sub.2 PSA.
M. S. A. Baksh, et.al. in "Lithium Type X-Zeolite as a Superior Sorbent for Air Separation" discloses the preparation of LiX-zeolite by ion exchange with commercial NaX-zeolite 8.times.12 mesh beads. The lithium exchange level is not set forth.
Japanese Kokai 2,153,818 discloses a zeolitic material for air separation having an A-zeolite structure comprising 40-85% and a kaolin content of 60-15%.
Those skilled in the art have generally believed that the addition of binder reduces the adsorptive properties of zeolitic containing materials. In the past, the trend has been to try to reduce the levels of binder from the typical 20% to as low as possible, often as low as 5%., while at the same time maintaining adequate crush strength. For example, Heinze in U.S. Pat. No. 3,356,450 states that it is advantageous to obtain hard formed zeolite particles with the lowest possible binder content to maintain high adsorption capacity. He discloses the use of a process which starts with molecular sieve granules bound with silicic acid, which are then treated with aqueous solutions containing alumina and alkali metal hydroxide, whereby the binder is converted to molecular sieve particles. The result is a practically binder-free shaped material with good abrasion resistance.
At the extreme of this trend toward reduced binder contents is the development of processes for preparing binderless bodies. Flank et al. (U.S. Pat. No. 4,818,508) teach the preparation of zeolites, particularly X, Y, and A, in massive bodies from calcined preforms made of controlled-particle-size kaolin-type clay. Kuznicki et al. (U.S. Pat. No. 4,603,040) teach the preparation of low silica X-zeolite (LSX) in the form of essentially binderless aggregates by reaction of calcined kaolin preforms in an aqueous solution of NAOH and KOH. GB 1,567,856 teaches a process for converting an extruded mixture of metakaolin and sodium hydroxide to A-zeolite. The advantage stated is that the method does not require the use of a binder such as clay, which usually reduces the activity of the molecular sieve by 15-20%.
Adsorbents produced from these binderless bodies are stated to have superior adsorptive properties when used for air separation. One such adsorbent is Ca low silica X-zeolite (CaLSX), prepared by Coe et al., (Coe, et al., "Molecularly Engineered, High-Performance Adsorbent: Self-Bound Low-Silica X Zeolite" in Perspectives in Molecular Sieve Science; Flank, W. H.; Whyte, Jr., T. E., Eds.; ACS Symposium Series 368; American Chemical Society: Washington, D.C., 1988; pp 478-491). "The self-bound LSX adsorbents do not have any binder to `dilute` the active component and lower the gas capacity." In addition, Coe et al. in U.S. Pat. No. 4,925,460 prepared chabazite from zeolite Y extrudate. They state, "This method produces a superior adsorbent, since adsorptive capacity decreases as binder content increases." These materials were converted to the Li form and used for separation of air, among other processes. Thirdly, Chao in U.S. Pat. No. 4,859,217 claims a process for selectively adsorbing N.sub.2 using X-zeolite having a framework Si/Al molar ratio not greater than 1.5 and having at least 88% of its AlO.sub.2 tetrahedral units associated with Li cations. He converted the bulk of the 20% binder in a zeolite "preform" agglomerate to X-zeolite crystals, obtaining essentially a binderless zeolite prior to ion exchanging into the Li form.
PSA processes for selectively adsorbing nitrogen from gas mixtures, such as air, comprise contacting the gas mixture with a zone containing an adsorbent such as one of those described above which is selective for the adsorption of nitrogen. Typically the zone is operated through a series of steps comprising: adsorption, during which the gas mixture contacts the adsorbent, nitrogen is selectively adsorbed and oxygen passes through the zone and can be recovered as product; depressurization during which the gas mixture contact is discontinued and the zone is reduced in pressure to desorb the nitrogen which can be recovered as product; and repressurization with air or oxygen product to the adsorption pressure.
The heat effects that occur during the adsorption and desorption steps can be of significance in the overall performance of PSA processes. Since the processes are essentially adiabatic, release of heat from the heat of adsorption increases the bed temperature during the adsorption step. Because of the higher temperature, adsorbate loading at the end of adsorption is lower than would be the case if the temperature did not increase. Likewise, during the desorption and optional purge steps, heat is required to desorb the adsorbate, decreasing the bed temperature. Because of the lower temperature, adsorbate loading at the end of desorption is higher than would be the case if the temperature did not decrease. These fluctuations in temperature reduce the working capacity of the adsorbent bed compared to the isothermal case.
The prior art has recognized that reduction of these temperature fluctuations can be beneficial when the adsorbate is strongly adsorbed with a high heat of adsorption. For example, Fuderer in U.S. Pat. No. 4,499,208 teaches the benefits to be obtained in using activated carbon doped with an inert inorganic material with a higher volumetric heat capacity than the carbon for strongly sorbed adsorbates at high partial pressures. He specifically claims the use of dense alumina with activated carbon. Fuderer mentions that molecular sieves having such inert materials could also be advantageous in various PSA separations for which such molecular sieves are well suited. However, he notes that the doped adsorbents of his invention are not applicable for advantageous use in all PSA separations. The preferred adsorbates are CO.sub.2 and more strongly sorbed adsorbates. There is no indication that doping would enhance the performance of air separation adsorbents, which adsorb N.sub.2 much more weakly than CO.sub.2. There is no suggestion that comparable or lower heat capacity diluents would be useful for PSA adsorbents.
Yang and Cen (R. T. Yang and P. L. Cen, Ind. Eng. Chem. Process Des. Dev., 1986, 25, 54-59) also demonstrated that high heat capacity inert additives resulted in substantial improvements in product purities and recoveries for bulk PSA separations of H.sub.2 /CH.sub.4 and H.sub.2 /CO mixtures using activated carbon adsorbents. Although the heats of adsorption of CH .sub.4 and CO on activated carbon are similar to the heat of adsorption of N.sub.2 on zeolites, the high pressures used in these separations (feed pressure of 21.4 atm) resulted in substantially larger temperature excursions in the bed than are observed for air separation. As in the reference of Fuderer, there is no indication that inert additives would enhance the performance of air separation adsorbents.
It is also notable that in both the case of Fuderer and that of Yang and Cen, the dopants or inert additives had higher heat capacities than the active adsorbent phase. There is no indication in the prior art that diluting a zeolitic adsorbent with a material that has heat capacity equal to or lower than the zeolite would result in a performance improvement.
In summary, there is no indication in the prior art that dilution of the zeolitic phase is beneficial for low heat processes, such as air separation. There is no indication that any temperature effect for air separation would be of sufficient magnitude that performance benefits resulting from dilution would overcome the detrimental effects of decreasing the specific isothermal adsorptive capacity (mmol/g) of the adsorbent zone. There is no indication that dilution of the zeolitic phase with a diluent with a lower heat capacity than the zeolitic phase will have a beneficial effect on air separation.